The present invention relates to the improvement of an amplifier, in particular, relates to a semiconductor amplifier which is used in VHF or higher frequency band.
FIG. 1 shows a blockdiagram of a prior semiconductor amplifier which can be used for a television transmitter in VHF or higher frequency band. In FIG. 1, a pair of semiconductor amplifiers 30 and 32 are connected in parallel since a single semiconductor amplifier can not provide enough output power. In order to assure the parallel operation of two semiconductor amplifiers, a power divider 10 and a power combiner 20 are utilized as shown in the figure. The input terminal(IN) which has the characteristic impedance Z.sub.0 is applied to the power divider 10, which provides in turn a pair of outputs. Each of the outputs of the power divider 10 has the characteristic impedance Z.sub.0 and is connected to the amplifier 30 or 32. Also, each output of the amplifier 30 or 32 is connected to the input of the power combiner 20, the output of which is connected to the output terminal(OUT). Of course, the characteristic impedance of the input terminals and output terminal of the power combiner 20 is Z.sub.0. Each of the power divider and the power combiner comprises a hybrid circuit 12 and a pair of circulators 14 and 16. The hybrid circuit 12 has an input terminal and a pair of output terminals, or an output terminal and a pair of input terminals, and all the terminals have the same characteristic impedance Z.sub.0. FIG. 2 is the example of the hybrid circuit, and is called the Wilkinson type hybrid circuit. The Wilkinson type circuit has legs P.sub.1 and P.sub.2, the length of which is .lambda./4, and these legs are connected to another leg P.sub.3. The terminals a, b, and c of the Wilkinson circuit are connected to an outside element as shown in FIG. 1. The resistance R is connected between the terminals b and c, for providing the in-phase signals between those terminals.
The circulators 14 and 16 in FIG. 1 function to provide a stable operation of the amplifiers 30 and 32. In particular, when there is something wrong with the outside circuit connected to the output terminal(OUT) of the present amplifier, the output power from the two semiconductor amplifiers will be reflected due to the mismatching of the characteristic impedance, and the reflected power is absorbed in the circulators 14 and/or 16.
Since all the terminals of the hybrid circuit 12 have the same characteristic impedance Z.sub.0, all the terminals of the circulators have the same characteristic impedance Z.sub.0, accordingly.
However, the amplifier shown in FIG. 1 has the disadvantage that the structure is complicated, in particular, the presence of the hybrid circuits makes the structure more complicated.